Lamellar barrier

Properties of the Lamellar Barrier — Effects of Penetration Enhancers. 17... 

A further paper [167] explains the lamellar thickness selection in the row model. The minimum thickness lmin is derived from the similation and found to be consistent with equilibrium results. The thickness deviation 81 = l — lmin is approximately constant with /. It is established that the model fulfills the criteria of a kinetic theory Firstly, a driving force term (proportional to 81) and a barrier term (proportional to /) are indentified. Secondly, the competition between the two terms leads to a maximum in growth rate (see Fig. 2.4) which is located at the average thickness l obtained by simulation. Further, the role of fluctuations becomes apparent when the dependence on the interaction energy e is investigated. Whereas downwards (i.e. decreasing l) fluctuations are approximately independent... 

H. R. Moghimi, B. W. Barry, and A. C. Williams. Stratum corneum and barrier performance A model lamellar structural approach. In R. L. Bronaugh and H. Maibach (eds.), Percutaneous Absorption—Drugs, Cosmetics, Mechanisms, Methodology, Marcel Dekker, New York, Basel, Hong Kong, 1999, pp. 515-553. 

A. P. M. Lavrijsen, J. A. Bouwstra, G. S. Gooris, A. Weerheim, H. E. Bodde, and M. Ponec. Reduced skin barrier function parallels abnormal stratum comeum lipid organization in patients with lamellar ichthyosis. J. Invest. Dermatol. 105 619-624 (1995). 

Landmann L (1986) Epidermal permeability barrier Transformation of lamellar granule-disks into intercellular sheets by a membrane-fusion process, a freeze-fracture study. J Invest Dermatol 87 202-209... 

Since A i is proportional to the supercooling, the LH theory predicts that as the crystallization temperature is lowered to a value when Eq. (1.95) is satisfied, the lamellar thickness would diverge. This is referred to as the 8L catastrophe. By taking reasonable experimental values for the various parameters and assuming t / = 1 (no barriers for attachment of each stem), the necessary supercooling for the appearance of this catastrophe is 55 K for polyethylene. This is not observed experimentally. To fix this discrepancy, vf/ is taken to be zero so that... 

The deposition of each stem of m repeat units can be treated [8,41] as a set of m equilibria. While this generalization appears to account for more local details, the general conclusions are the same as in the LH theory. In realistic situations, we expect nonsequential deposition of repeat units into various stems. These partially formed stems will then sort out through entropic barriers to attain the lamellar thickness. We remrn to this issue in Section VII. 

The minimum in F(L) is observed to be near L/ q — 9 for aU chain lengths examined. It is to be noted that this minimum is the global minimum and the barrier between this state and other thicker lamellae increases prohibitively as the thickness increases. These simulations strongly suggest that a lamellar thickness that is much smaller than the extended chain thickness is actually an equilibrium result. 

The most superficial layer of skin is the stratum comeum (SC), which consists of terminally differentiated keratinocytes (comeocytes) that originate from actively proliferating keratinocytes in lower epidermis (basale, spinosum, and granulosum cells), and contain a lamellar lipid layer secreted from lamellar bodies (Fig. 7a). Flydration of the SC is an important determinant of skin appearance and physical properties, and depends on a number of factors including the external humidity, and its structure, lipid/protein composition, barrier properties, and concentration of water-retaining osmolytes (natural moisturizing factors, NMFs) including free amino acids, ions, and other small solutes. 

When the microencapsulated liposomes are left untreated the lipid bilayer provides a barrier to diffusion through which the entrapped protein does not pass until the liposomes gradually become leaky, primarily due to oxidation of the phospholipid side chains. This mechanism results in a delayed release. Triton or sonic treatment of the microencapsulated liposomes provide pulsed re ease. Since both detergent and sonication disrupt lipid bi ayers, the mechanism by which pulsed release is achieved may be that these stimuli initially disrupt the liposomes and then the lipid reforms around some of the protein solution inside the capsule, possibly in an altered lamellar form alternatively, the treatment could disrupt only the more susceptible liposomes, leading to two phases of release, first from the freed protein and later from protein that remained liposome-entrapped. 

In principle, all lamellar minerals may be used as barrier pigments, e.g., micaceous iron oxide [5.167]-[5.169], layer silicates (mica), linear polymeric silicates (wollas-tonite), and talc [5.170], However, untreated mica and talc are not very suitable because they are highly permeable to water [5.57]. The surface can be modified with, for example, silanes or titanates, to reduce water permeability and improve adhesion... 

The stratum granulosum is the most superficial cell layer of the viable epidermis and contains highly differentiated keratinocytes. The lamellar bodies, which have been formed in the stratum spinosum, migrate to the apical periphery of the uppermost granular cells and eventually fuse with the membrane of the keratinocyte. Via exocytosis their content is extruded into the intercellular spaces at the stratum granulosum-stratum corneum interface. The lipids derived from the lamellar bodies are essential for the formation of the stratum corneum barrier. 

There are several genetic skin diseases with known defects in the lipid metabolism. Atopic dermatitis, lamellar ichthyosis, and psoriasis have been the most widely studied with respect to epidermal barrier function and alterations in the lipid profile. Deviations in the lipid profile have been linked with an impaired stratum corneum barrier function. Atopic dermatitis is characterized by inflammatory, dry and easily irritable skin, and overall reduced ceramide levels in the stratum corneum [58-60]. In particular a significant decrease in the ceramide 1 level is observed, whereas the levels of oleate that is esterified to ceramide 1 are elevated [59]. Both aberrations may be responsible for the reduced order of the lamellar phases as observed with freeze fracture electron microscopy [61]. It has further been established that, in comparison to healthy stratum corneum, the fraction of lipids forming a hexagonal packing is increased [61]. A recent study reveals that the level of free fatty acids... 

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